Process for electrochemical fluorination

ABSTRACT

CIRCULATION OF AMIXTURE OF COMPOSITIOM TO BE FLUORINATED AND ANHYDROUS AND HYDROFLUORIC ACID AS ELECTROLYTE THROUGH A COOLING ZONE, AN ELECTROLYTIC CELL AND A RELATIVELY LARGE STORAGE ZONE WHILE REMOVING INSOLUBLE FLUORINATION PRODUCTS FROM THE ELECTROLYTE BEFORE A SECOND PASSAGE THROUGH SAID CELL.

Aug. 21, 1973 P. voss ETAL 3,753,876

PROCESS FOR ELECTROCHEMICAL FLUORINATION Filed Feb. 10, 1972 United States Patent "ice 3,753,876 PROCESS FOR ELECTROCHEMICAL FLUORINATION Peter Voss, Leverkusen, Hans Niederprum, Monheim, Rhineland; Gustav Kaule, Cologne, and Rudiger Trupp, Leverkusen, Germany, assignors to Bayer Aktiengesellschaft, Leverkusen, Germany Filed Feb. 10, 1972, Ser. No. 225,075 Claims priority, application Germany, Feb. 13,1971, P 21 06 870.2 Int. Cl. B01k 3/00; C22d 1/02; (1231) 5 68 U.S. Cl. 204-59 R 7 Claims ABSTRACT OF THE DISCLOSURE Circulation of a mixture of composition to be fluorinated and anhydrous hydrofluoric acid as electrolyte through a cooling zone, an electrolytic cell and a relatively large storage zone while removing insoluble fluorination products from the electrolyte before a second passage through said cell.

The invention relates to a process and apparatus for electrochemical fluorination of organic and inorganic substances in anhydrous hydrofluoric acid. The whole concept of the invention and the apparatus used difler radically from any previously described electrofluorination cells (J. Burdon, J. C. Tallow: Advances in Fluorine Chemistry, vol. 1, p.p. 129-165, London, 1960; Forche, Houbeu- Weyl: Methoden der Qrganischen Chemie, vol. V/3, pp. 38-53, Stuttgart, 1962; S. Nagase, Fluorine Chemistry Reviews 1 (1), 77-106, 1967; N. Watanabe, Denki Kagaku 36 [1968], pp. 172-186 and 264-269.

All the electrochemical fluorination processes previously described are so-called one pot processes, that is to say the electrode block, consisting of alternating cathode and anode plates, dips into the whole quantity of electrolyte. This process has distinct disadvantages. It is a well known object in technical fluorination cells to use as large a volume of electrolyte as possible because a large quantity of material used over prolonged periods of time ensures a substantially constant concentration of electrolyte and therefore reproducible conditions.

In the one pot processes previously known, the use of a large volume of electrolyte entails uncontrolled loss of heat and inefiicient diffusion of material at the anode surface. Since, however, temperature and diffusion materially influence the yield of product and high temperatures between electrode plates promote formation of polymerisation products and hence resinification of the electrodes, rapid removal of heat in the electrode block is essential. Cooling of the electrolyte cells has previously been effected exclusively by means of cooling jackets placed round the cells or cooling coils introduced into the vessel of the cells or by condensation of hydrofluoric acid carried into the cooling systems connected to the cells. In most of the processes described, the removal of heat in the electrode block and the transport of the substance which is to be fluorinated to the anode are effected by a natural circulation which is brought about by the convection of heat and by the circulating effect of hydrogen formed at the cathode or of gaseous fluorination products which may be formed at the anode. Although the method which has already been proposed of supplying gaseous starting materials either through a sieve plate provided below the electrode block (Chemie-Ing.-Technik 36, No. 1, pp. 9-14, 1964; Dutch patent specification No. 6,814,889) or Patented Aug. 21, 1973 through porous anodes (British patent specification No. 740,723; German Auslegeschrift No. 1,040,009; German Oflenlegungsschrift No. 1,803,893) provides for more efficient removal of heat between the electrode plates and rapid transport of the unfluorinated or partly fluorinated starting material to the anode surface, it is in effect advantageous only in cases where the starting materials or end products are gaseous. The method of blowing nitrogen through the electrode block also provides higher current yields and higher yields of material due to the more efiicient removal of heat and the fact that the reaction takes place under conditions of controlled dilfusion (I. Kadija and D. Drazic, Institut fiir Chemie and Metallurgie, Belgrade, Jugoslavia; Abstracts 3rd European Symposium on fluorine, Sept. 14-17, 1970, AiX-en-Provence, France), but this method would appear to be too costly if carried out on a technical scale on account of the high vapour pressure of hydrofluoric acid (0 C3320 mm. Hg/20 0.2160 mm. Hg/40 (1251.9 mm. Hg)

and the high losses which this entails. Another possibility of improving the diffusion and removal of heat is provided by the use of rotating anodes (US. patent specification No. 2,806,817) but the construction of rotating anodes (240 to 1000 revs/min.) in production cells, of the order of 40,000 amperes constitutes a special technical problem. The use of stirrers in the electrolytic cell also results in improved yields but has the disadvantage that the resulting liquid, perfluorinated products, which are no longer soluble in hydrofluoric acid, cannot settle at the bottom of the cell but remain suspended in the hydrofluoric acid and are therefore subject to continued attack from the anode and moreover cannot be removed continuously (N. Kisaki, S. Mabuchi, T. Sakomura, Denki kagaku, vol. 34, p. 24, 1966).

The process according to this invention for electrochemical fluorination has distinct advantages over all the processes previously known and described in the literature. It is distinguished by the fact that the electrolyte is continuously pumped from a large storage vessel through an electrode block in such a manner that the resulting perfiuorinated product which is insoluble in hydrofluoric acid is separated, e.g. by interposing tranquillizing zones which are either arranged separately or situated in the storage vessel, so that this insoluble product only passes once through the electrolytic cell. The capacity of the storage vessel should be at least the volume of electrolyte said cell may contain and is preferably from 4 to 6 timeslarger, but without any limitation of the upper value. The positive circulation of electrolyte provided by this process effects rapid removal of heat from the electrode block and constant renewal and rapid transport of the product which is fluorinated at the anode surface, with the result that higher current yields and higher yields of material are achieved, as will be explained in the examples which follow.

In addition, the time of operation of the cells is considerably increased because the extent to which unwanted polymerisation and resinification reactions occur at the electrodes is substantially reduced.

It is also possible to use an electrolytic cell unit consisting of more than one individual cell. The individual cells of such an electrolytic unit may be connected either parallel or in series.

A preferred method of carrying out the process is de- (1) container for anhydrous hydrofluoric acid; (2) storage vessel for electrolyte;

(3) immersion pump;

(4) flow rate regulator;

(5) cooler;

(6) electrolytic cell;

(7) electrode block;

(8) condenser;

(l) electrolyte inflow;

( 1 l) electrolyte outflow;

( l2) condensate; and

(13) gaseous products taken to KOH scrubber.

Hydrofluoric acid is forced from a container 1 into the storage vessel 2 together with nitrogen. Inorganic or or ganic solid, liquid or gaseous materials which are to be fluorinated are fed continuously or intermittently into the storage vessel through suitable devices in its cover. An immersion pump 3 forces the electrolyte through a cooling system via a flow rate regulator 4, and from there into an electrolytic cell 6 arranged above the cooling system, the flow of electrolyte into the electrolytic cell being preferably so arranged that the whole quantity of electrolyte must flow through the electrode block 7 from below. The electrolyte level in the cell is controlled by an overflow dam. The fluorinated compounds produced by electrolysis are fed into the storage vessel 2 together with circulated electrolyte. Liquid and solid perfluorinated reaction products may be removed continuously or intermittently from the bottom of the storage vessel. Gaseous products, especially hydrogen and hydrofluoric acid, are passed through a condenser 8 in known manner and from there to a KOH gas scrubber. The condensate obtained is returned to the storage vessel 2. The rates of circulation and cooling are preferably so regulated that a temperature of to 15 C. is maintained in the electrolytic cell.

The following example illustrates the above described advantages of this process with reference to the electrochemical fluorination of butadiene sulphone:

Electrolyte was pumped from a storage vessel containing 24.0 kg. of butadiene sulphone dissolved in 120 litres of anhydrous hydrofluoric acid into the electrolytic vessel proper via a throughflow cooler at an average flow rate of 400 l./hr. The electrolytic cell had a capacity of 27.5 1. The electrode block consisted of 15 anodes and 16 cathodes. The eflective anode surface area was 12,390 cm. corresponding to a current density of 0.008 a./cm. when the current supply was 100 amps. A total quantity of 100.0 kg. of butadiene sulphone was employed at an average concentration of about 10%. The current consumption after 5014.40 hours was 459,774 ampere hours, i.e. the average load was 91.8 amps. The reaction product collected at the bottom of the storage vessel and could be removed through a pump. The average cell temperature was +5 C. The voltage varied during the whole experiment between 6.7 and 7.0 volt.

The total amount of product removed was 163.5 kg. which according to gas chromatographic analysis contained 85.5% of perfluorobutyl sulphonyl fluoride. This corresponds to a yield of 139.8 kg. of pure C F SO F.

The material yield was 54.6% of the theoretical. The current yield was 42.9% of the theoretical.

(1b) Fluorination by the stationary system (comparison example) The electrolytic cell in the operative state contained 27.5 i. of hydrofluoric acid. The electrode block consisted of 31 nickel plates (15 anodes and 16 cathodes). The effective anode surface was 12,390 cm. corresponding to a current density of 0.008 a./cm. at a load of amps. The cell was charged with 5.500 kg. of butadiene sulphone and filled up with hydrofluoric acid. This corresponded to an electrolyte concentration of 20%. Electrolysis was carried out at an average temperature of 0 C. Hydrofluoric acid and butadiene sulphone were added as required so that the average electrolyte concentration was about 10%. The conductivity of butadiene sulphone in HF was so high that no other electrolyte needed to be added.

67.690 kg. of butadiene sulphone were fluorinated over a period of 3448.35 hours. The current consumption at an average current intensity of 90.6 amps, and an average voltage of 5.85 v. was 312.421 amp hours. A total of 81.069 kg. of reaction product having a gas chromatic graphic purity of 82.30% was discharged from the bottom of the cell. This corresponds to a yield of 66.722 kg. of pure perfluorobutyl-sulphonylfluoride.

The material yield was 38.5% of the theoretical. The current yield was 30.1% of the theoretical.

What is claimed is:

1. In the process for the electrochemical fluorination of an organic or inorganic composition, the improvement which com-prises serially circulating a mixture comprising the composition to be fluorinated and anhydrous hydrofluoric acid as electrolyte through a zone which cools the same, an electrolytic cell operating under electrolyzing conditions and a storage zone containing a volume of electrolyte at least of the same volume of electrolyte contained in said electrolytic cell while removing hydrofluoric acid insoluble products produced in said cell from said electrolyte before repassage thereof through said cell.

2. The process of claim 1 wherein said electrolyte is passed upwardly through said electrolytic cell.

3. The process of claim 1 wherein the rate of circulating said electrolyte and the rate of said cooling are adjustecl to obtain a temperature of -l0 to 15 C. in said electrolytic cell.

4. The process of claim 1 wherein hydrofluoric acid insoluble fluorinated reaction products are removed from the bottom of said storage zone.

5. The process of claim 1 wherein said volume of electrolyte in said storage zone is 4 to 6 times the volume of electrolyte contained in said electrolytic cell.

6. The process of claim 1, wherein said electrolytic cell consists of more than one individual cell unit which are connected in parallel or in series.

7. The process of claim 1 wherein the composition to be fluorinated is butadiene sulphone.

References Cited UNITED STATES PATENTS 3,663,380 5/1972 Mills 204-59 R 3,650,917 3/ 1972 Ruehlen 204-59 R 3,620,941 11/1971 Ruehlen 204-59 R 2,697,726 12/1954 Silley et al. 20481 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204275 

